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<div class="titlepage"><div><div><h5 class="title">
<a name="spirit.karma.reference.directive.buffer"></a><a class="link" href="buffer.html" title="Generator Directive for Temporary Output Buffering (buffer[])">Generator
          Directive for Temporary Output Buffering (<code class="computeroutput"><span class="identifier">buffer</span><span class="special">[]</span></code>)</a>
</h5></div></div></div>
<h6>
<a name="spirit.karma.reference.directive.buffer.h0"></a>
            <span class="phrase"><a name="spirit.karma.reference.directive.buffer.description"></a></span><a class="link" href="buffer.html#spirit.karma.reference.directive.buffer.description">Description</a>
          </h6>
<p>
            All generator components (except the <a class="link" href="../operator/alternative.html" title="Alternative Generator (a | b)">Alternative
            (<code class="computeroutput"><span class="special">|</span></code>)</a> generator) pass
            their generated output directly to the underlying output stream. If a
            generator fails halfway through, the output generated so far is not 'rolled
            back'. The buffering generator directive allows to avoid this unwanted
            output to be generated. It temporarily redirects the output produced
            by the embedded generator into a buffer. This buffer is flushed to the
            underlying stream only after the embedded generator succeeded, but is
            discarded otherwise.
          </p>
<h6>
<a name="spirit.karma.reference.directive.buffer.h1"></a>
            <span class="phrase"><a name="spirit.karma.reference.directive.buffer.header"></a></span><a class="link" href="buffer.html#spirit.karma.reference.directive.buffer.header">Header</a>
          </h6>
<pre class="programlisting"><span class="comment">// forwards to &lt;boost/spirit/home/karma/directive/buffer.hpp&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">karma_buffer</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
            Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>.
          </p>
<div class="informaltable"><table class="table">
<colgroup><col></colgroup>
<thead><tr><th>
                    <p>
                      Name
                    </p>
                  </th></tr></thead>
<tbody><tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">buffer</span> <span class="comment">//
                      alias: boost::spirit::karma::buffer</span></code>
                    </p>
                  </td></tr></tbody>
</table></div>
<h6>
<a name="spirit.karma.reference.directive.buffer.h2"></a>
            <span class="phrase"><a name="spirit.karma.reference.directive.buffer.model_of"></a></span><a class="link" href="buffer.html#spirit.karma.reference.directive.buffer.model_of">Model of</a>
          </h6>
<div class="blockquote"><blockquote class="blockquote"><p>
              <a class="link" href="../generator_concepts/unarygenerator.html" title="UnaryGenerator"><code class="computeroutput"><span class="identifier">UnaryGenerator</span></code></a>
            </p></blockquote></div>
<div class="variablelist">
<p class="title"><b>Notation</b></p>
<dl class="variablelist">
<dt><span class="term"><code class="computeroutput"><span class="identifier">a</span></code></span></dt>
<dd><p>
                  A generator object
                </p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">A</span></code></span></dt>
<dd><p>
                  Attribute type of generator <code class="computeroutput"><span class="identifier">a</span></code>
                </p></dd>
</dl>
</div>
<h6>
<a name="spirit.karma.reference.directive.buffer.h3"></a>
            <span class="phrase"><a name="spirit.karma.reference.directive.buffer.expression_semantics"></a></span><a class="link" href="buffer.html#spirit.karma.reference.directive.buffer.expression_semantics">Expression
            Semantics</a>
          </h6>
<p>
            Semantics of an expression is defined only where it differs from, or
            is not defined in <a class="link" href="../generator_concepts/unarygenerator.html" title="UnaryGenerator"><code class="computeroutput"><span class="identifier">UnaryGenerator</span></code></a>.
          </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Expression
                    </p>
                  </th>
<th>
                    <p>
                      Semantics
                    </p>
                  </th>
</tr></thead>
<tbody><tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">buffer</span><span class="special">[</span><span class="identifier">a</span><span class="special">]</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      The embedded generator <code class="computeroutput"><span class="identifier">a</span></code>
                      is invoked but its output is temporarily intercepted and stored
                      in an internal buffer. If <code class="computeroutput"><span class="identifier">a</span></code>
                      succeeds the buffer content is flushed to the underlying output
                      stream, otherwise the buffer content is discarded. The buffer
                      directive succeeds as long as the embedded generator succeeded
                      (unless the underlying output stream reports an error).
                    </p>
                  </td>
</tr></tbody>
</table></div>
<div class="tip"><table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../images/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top"><p>
              If you want to make the buffered generator succeed regardless of the
              outcome of the embedded generator, simply wrap the <code class="computeroutput"><span class="identifier">buffer</span><span class="special">[</span><span class="identifier">a</span><span class="special">]</span></code> into an additional optional: <code class="computeroutput"><span class="special">-</span><span class="identifier">buffer</span><span class="special">[</span><span class="identifier">a</span><span class="special">]</span></code> (see <a class="link" href="../operator/optional.html" title="Optional Generator (-a)">Optional
              (unary <code class="computeroutput"><span class="special">-</span></code>)</a>).
            </p></td></tr>
</table></div>
<h6>
<a name="spirit.karma.reference.directive.buffer.h4"></a>
            <span class="phrase"><a name="spirit.karma.reference.directive.buffer.attributes"></a></span><a class="link" href="buffer.html#spirit.karma.reference.directive.buffer.attributes">Attributes</a>
          </h6>
<p>
            See <a class="link" href="../../quick_reference/compound_attribute_rules.html#spirit.karma.quick_reference.compound_attribute_rules.notation">Compound
            Attribute Notation</a>.
          </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Expression
                    </p>
                  </th>
<th>
                    <p>
                      Attribute
                    </p>
                  </th>
</tr></thead>
<tbody><tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">buffer</span><span class="special">[</span><span class="identifier">a</span><span class="special">]</span></code>
                    </p>
                  </td>
<td>
                    <p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span> <span class="special">--&gt;</span> <span class="identifier">buffer</span><span class="special">[</span><span class="identifier">a</span><span class="special">]:</span> <span class="identifier">A</span>
<span class="identifier">a</span><span class="special">:</span> <span class="identifier">Unused</span> <span class="special">--&gt;</span> <span class="identifier">buffer</span><span class="special">[</span><span class="identifier">a</span><span class="special">]:</span> <span class="identifier">Unused</span></pre>
<p>
                    </p>
                  </td>
</tr></tbody>
</table></div>
<h6>
<a name="spirit.karma.reference.directive.buffer.h5"></a>
            <span class="phrase"><a name="spirit.karma.reference.directive.buffer.complexity"></a></span><a class="link" href="buffer.html#spirit.karma.reference.directive.buffer.complexity">Complexity</a>
          </h6>
<div class="blockquote"><blockquote class="blockquote"><p>
              The overall complexity of the buffering generator directive is defined
              by the complexity of its embedded generator. The complexity of the
              buffering directive generator itself is O(N), where N is the number
              of characters buffered.
            </p></blockquote></div>
<h6>
<a name="spirit.karma.reference.directive.buffer.h6"></a>
            <span class="phrase"><a name="spirit.karma.reference.directive.buffer.example"></a></span><a class="link" href="buffer.html#spirit.karma.reference.directive.buffer.example">Example</a>
          </h6>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              The test harness for the example(s) below is presented in the <a class="link" href="../basics.html#spirit.karma.reference.basics.examples">Basics Examples</a>
              section.
            </p></td></tr>
</table></div>
<p>
            Some includes:
          </p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">karma</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">support_utree</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">phoenix_core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">phoenix_operator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">std_pair</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span>
</pre>
<p>
          </p>
<p>
            Some using declarations:
          </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">karma</span><span class="special">::</span><span class="identifier">double_</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">karma</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">;</span>
</pre>
<p>
          </p>
<p>
            Basic usage of a buffering generator directive. It shows how the partial
            output generated in the first example does not show up in the generated
            output as the plus generator fails (no data is available, see <a class="link" href="../operator/plus.html" title="Plus Generator (+a)">Plus
            (unary <code class="computeroutput"><span class="special">+</span></code>)</a>).
          </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">v</span><span class="special">;</span>                <span class="comment">// empty container</span>
<span class="identifier">test_generator_attr</span><span class="special">(</span><span class="string">""</span><span class="special">,</span> <span class="special">-</span><span class="identifier">buffer</span><span class="special">[</span><span class="char">'['</span> <span class="special">&lt;&lt;</span> <span class="special">+</span><span class="identifier">double_</span> <span class="special">&lt;&lt;</span> <span class="char">']'</span><span class="special">],</span> <span class="identifier">v</span><span class="special">);</span>

<span class="identifier">v</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">1.0</span><span class="special">);</span>                     <span class="comment">// now, fill the container</span>
<span class="identifier">v</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">2.0</span><span class="special">);</span>
<span class="identifier">test_generator_attr</span><span class="special">(</span><span class="string">"[1.02.0]"</span><span class="special">,</span> <span class="identifier">buffer</span><span class="special">[</span><span class="char">'['</span> <span class="special">&lt;&lt;</span> <span class="special">+</span><span class="identifier">double_</span> <span class="special">&lt;&lt;</span> <span class="char">']'</span><span class="special">],</span> <span class="identifier">v</span><span class="special">);</span>
</pre>
<p>
          </p>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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